Alkali metal silicate developers have been extensively used to develop photosensitive lithographic printing plates. In the photosensitive layer of a conventional photosensitive lithographic printing plate, ortho-quinoneazide compounds are used in combination with novolak resin and an aqueous alkaline silicate solution capable of dissolving novolak resin is used as a developer. A developer has a pH of about 13 to dissolve the novolak resin of the photosensitive layer so that silicates exhibiting superior buffer capacity at a pH near 13 have been allowed to be contained in the developer to enhance stability, as described in, for example, JP-A No. 8-160633 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication).
Such a developer is liquid and so heavy, imposing a burden on the person handling them so that there have been widely proposed concentrated developers having a reduced water content. However, silicate-containing developers tend to easily solidify with reduction of the water content and even when re-diluted with water again, it is not easily dissolved in water, leading to insufficient concentration. To overcome such a problem, solidifying the silicate-containing developer in a granular form was proposed, as described in JP-A Nos. 5-142786 (pages 1–8), 6-266062 (pages 1–5) and 7-13341 (page 8). However, it was difficult to achieve stable solidification of the silicate, resulting in increased manufacturing costs and being of no practical advantage.
In a photopolymerization type photosensitive layer containing ethylenically unsaturated compounds, as represented by a photopolymer type CTP plate, on the other hand, novolak resin is not generally used, making it possible to design a developer at a pH of less than 12.5 and it has also no need to use a silicate as a pH buffer, rendering it possible to design a developer containing no silicate. In such developers described in the disclosure (for example, JP-A No. 2002-251019, page 34), there has not been proposed solidification of a developer component or a concentrate having a relatively high concentration factor. A developer for use in a photopolymer type CTP plate generally needs a higher developer-replenishing rate than conventional photosensitive lithographic printing plates which contain ortho-quinoneazide, and thus a lightweight developer has been desired.